Municipal solid waste incineration is a common and effective method of reducing the volume of solid waste while recovering energy. The byproduct of the incineration process is an incinerator fly ash, in which several volatile metals and organic compounds (e.g., dioxins and furans) are concentrated. Incinerator fly ash that is part of a waste stream captured from flue gas by pollution control devices usually has higher toxicity levels than incinerator fly ash collected at the bottom of a combustion chamber. The incinerator fly ash is classified as a hazardous waste because the heavy metals and other toxic compounds found in the incinerator fly ash may be easily leached into the ground or released into the atmosphere. This poses a potential health and environmental danger.
Geopolymers are synthesized from source materials that are rich in silica and alumina, such as coal fly ash. Geopolymers result from a chemical reaction where the silica and alumina molecules from the source materials react under highly alkaline conditions. The outcome of the reaction is an amorphous 3D network of Si and Al atoms linked by 0 atoms in a four-fold coordination similar to the one exhibited by zeolites. The positive ion (Na+ or K+) provided by the activator solution serves to balance the negative charge generated by having Al3+ atoms in a four-fold coordination. It has been reported that heavy metals may become encapsulated and immobilized within the geopolymer 3D network. While the mechanism by which heavy metal immobilization occurs is not yet fully understood, it has been proposed that initially metal ions are taken into the geopolymer network, subsequently they are bound into the structure in charge-balancing roles and finally, the remaining metal ions are physically encapsulated in the binder.